Github user AnthonyTruchet commented on the issue:
https://github.com/apache/spark/pull/16038
@mridulm we don't know how to monitor the size of the serialize task. Sure
it would not 10MB due to all those zeros. But we nonetheless measure a
significant increase in performance and (more importantly) in stability when
using the workaround and our custom TreeAggregatorWithZeroGenerator
@srowen when the density of a Sparse Vector increases it became *very*
inefficient, not only because it is using almost twice the size of memory, but
mainly because you fragment memory access on overload the GC :(
See #16078 for a generic wrapper around treeAggregate with uses Option[U]
do denote the zero of U by None and generate a full representation by need when
calling seqOp or comboOp.

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